rxrpc: Provide queuing helper functions

Provide queueing helper functions so that the queueing of local and
connection objects can be fixed later.

The issue is that a ref on the object needs to be passed to the work queue,
but the act of queueing the object may fail because the object is already
queued.  Testing the queuedness of an object before hand doesn't work
because there can be a race with someone else trying to queue it.  What
will have to be done is to adjust the refcount depending on the result of
the queue operation.

Signed-off-by: David Howells <dhowells@redhat.com>